When I export my simple project with flat and non-triangular planes into an .obj file and viewed through Windows 3D viewer, the triangulation appears to be glitched, with faces extending to where they shouldn’t be and multiple planes flickering on a flat plane where there should be one flat plane only. Before exporting, the project contains no hidden geometry and all planes are flat when viewed in SU. Is manually triangulating the flat planes my only way to solve this problem?
Have you tried the OBJ file in other applications than the viewer you use? Are you using the “Triangulate all faces” option when exporting from SketchUp? I haven’t used the OBJ format for years but when I did, the applications I tried to open them with usually didn’t support polyfaces and displayed artifacts like you describe unless the triangulation option was chosen in the export options. Especially bad were faces that contained holes.